Heat the olive oil in a large frying pan and over a medium heat, cook the onions for 5-6 minutes until softened. Stir occasionally. Add in the artichokes with 80ml of water and braise over a gentle heat for 6 minutes with the lid on.
Roll out the pastry on a lightly floured surface and use to line a 25cm diameter loose bottom tart tin. Chill in the freezer for 10 minutes.
Fill the tart tin with baking paper and baking beans, place on a baking tray and cook in the middle of a preheated oven at 180º for 15 minutes. Once ready, remove the paper and beans and set aside to cool.
In the meantime blanch the spinach by putting it in a colander over the sink and pouring boiling water over it. Squeeze out the excess water and set aside.
In a large bowl lightly beat the eggs, add in the cheeses the spinach and season with salt and pepper. Mix well. Stir in the artichokes and onion mixture and pour into the pastry case, spreading it out evenly.
Bake in the middle of the oven for approx 30 minutes or until the filling is just set throughout.
Allow to cool in the tin for 15 minutes, then remove and transfer to a large plate. Cut into slices and serve warm with your favourite salad.